The invention relates to a method for regenerating moisture-laden process air as well as to an arrangement for carrying out the method.
In numerous manufacturing processes, especially when processing plastics, the starting materials and/or the intermediate products must be dried before they are processed further. At the same time, the process air, used for the drying process, accumulates moisture, which was withdrawn from the starting materials and/or the intermediate products. Basically, the so-resulting moisture-laden warm process air could be discharged untreated to the environment and replaced by fresh air. However, such a solution is incompatible with conserving energy and therefore also excluded for reasons of costs.
When plastic granulates are processed, it is necessary, as is already mentioned above, to dry the plastic granulates before they are processed or processed further. This is done by means of warm, dry air (process air), which flows through the plastic granulates in a container, provided for this purpose, takes up the moisture to begin with and then subsequently must be freed once again from the moisture taken up.
For this purpose, the process air, in which the moisture has accumulated, is subjected to a regeneration process for removing this moisture from the drying cartridges filled with molecular sieves, by which the moisture of the process air, which is carried along, is absorbed.
Basically, several such drying cartridges are used, which are connected in parallel. While at least one of the drying cartridges is operating in the drying phase, the absorbed moisture is removed at the same time from one or more drying cartridges, which are connected in parallel, so that these cartridges are prepared for a new drying phase.
It is, for example, known from the state of the art that the regeneration of the drying cartridges may be conducted according to the so-called co-current principle, according to which the direction of flow in the drying cartridges is the same in the drying phase for the process air and the regeneration phase of the drying cartridge.
In the case of the so-called counter-current principle, the direction of flow during the regeneration of the drying cartridges is opposite to that during the drying process.
The advantage of the counter-current principle lies in the distinctly lower energy consumption during the regeneration. While the drying cartridge is absorbing moisture from the process air during the drying process, it is moistened continuously in the direction of flow. A front, separating the moist region from the dry cartridge, migrates in the direction of flow. In good time, before this “front”, so formed and migrating through the drying cartridge, has reached the end of the drying cartridge, the flow of the process air must be switched over to a different drying cartridge while a safety or buffer zone is maintained. By these means, it is ensured that a dry zone is available without temporal interruption and that the drying process can, accordingly, take place continuously without interruption.
For the countercurrent regeneration, the still-remaining dry region of the drying cartridge is now used as the starting point, from which the adjoining, moistened regions of the drying cartridge are dried once again continuously in a direction opposite to the preceding moistening of the drying cartridge. The front, separating the moist from the dry region, now migrates back opposite to its original direction of movement.
In the case of the previously described co-current regeneration, the front, separating the moist from the dry region, rolls over the region of the drying cartridge, which initially remains dry, and initially still absorbs moisture from the region of the drying cartridge, preceding the front separating the moist from a dry region.
Finally, the “by-pass method” is also used in some cases, for which the regeneration of the drying cartridge takes place at all times with a partial amount of the process air. For this purpose, 15 to 20% of the process air is diverted from the main stream, heated to about 220° to 300° C. and supplied to the regenerating drying cartridge. At the end of the drying process, the heating, assigned to the drying cartridge, which is to be regenerated, is switched off and the regenerated drying cartridge is cooled to about 60° C. with the diverted stream of process air. Only when the temperature has been lowered to such a level, is the molecular sieve in the drying cartridge, which is to be regenerated, once again fully effective. At the same time, however, a constantly vented partial amount of the process air must be taken up in the cycle as fresh air, so that the process air, to a certain extent, is additionally loaded with moisture and the drying cartridge in the drying process is saturated more quickly with moisture.
As is evident from the state of the art, there have been numerous efforts to improve the functionality and efficiency of the regeneration.